A typical automotive vehicle is equipped with disc brake systems for the purpose of stopping the vehicle. These disc brake systems are located at the front axle wheel assemblies and/or at the rear wheel assembly. Each wheel assembly typically includes a wheel hub, a rotor and a bearing. The bearing may further engage a knuckle that in turn may be connected to the vehicle. The disc brake rotor is comprised of a circular metal disc having opposed braking surfaces that are clamped by the brake pads to exert a braking effect. The rotor is attached to a wheel hub.
Performance of the braking system is related to the dimensional characteristics of the rotor and the wheel hub surface abutting the rotor. Any run-out variation in the wheel hub surface will cause lateral run-out or lateral deflection in the rotor. Lateral disc run-out refers to a lateral deviation of the planar surface of a rotor along a plane perpendicular to the longitudinal axis of rotation of the rotor. Because the rotor is designed to operate in a precise plane normal to the axis of the wheel, even slight run-out variations of the wheel hub is problematic. Similarly, the radial run-out of the outer edges of the braking surfaces need to be controlled to ensure that the brake pads engage as much of the available rotor-braking surface as possible without overlapping the edges of the rotor. If run-out is not controlled it can cause premature failure of the brake lining due to uneven wear which requires premature replacement of the brake lining at an increased expense. Further, problems due to run-out include, brake judder, steering wheel “nibble” and pedal pulses felt by the user, and warped rotors which result in brake noise and uneven stopping. However, manufacturers have faced difficulties in achieving enhanced control over these tolerances due to the influence of several factors.
One factor that frequently contributes to lateral run-out is variation in the processes that are used to machine the flange surface of the wheel hub. For example, the outer and inner flange surfaces of the wheel hub may be individually machined causing uneven deviation of the planar surface of the wheel hub.
Another factor that contributes to run-out is the stack-up of the individual components in a wheel assembly, i.e., their combined tolerances. While the tolerances of each part can be reduced when they are separately machined, when the parts are assembled, the combined tolerances stack up, causing run-out that is still relatively significant. Tolerance stacking may also be caused by variation in the turning processes that are used to machine the flange surface, when the wheel hub is individually machined, in an effort to make it flat with respect to the rotor. Further, the installation and press condition of the wheel bolts, the assembly process of the wheel assembly, and improperly pre-loaded bearings, can all cause misalignment of the hub surface with respect to the rotor and thus cause unacceptable run-out.
Lateral run-out may also be caused by the insertion and/or press-fitting of the wheel bolts to the flange after the flange surface of the wheel hub has been finished or machined. When the wheel bolts are press-fitted or tightened to the flange surface, the force causes the peripheral areas immediately around the wheel bolts to deform on the flange surface. Consequently, this deformation causes the surface of the flange to deform and deviate from the necessary planar surface of the wheel hub, causing lateral run-out.
The process of pressing or assembling the hub to the bearing is another possible factor that causes lateral run out. When the bearing is assembled to the hub, additional run-out variation may be introduced to the rotor mounting face of the hub. Additionally, removal of the bearing and/or reassembling the bearing to the wheel hub after machining the wheel hub can re-introduce lateral run-out variation.
Therefore, a need exists for an apparatus and a method for machining the wheel hub to eliminate the lateral run-out after the wheel bolts or bearing have been attached and to evenly machine the flange surface without significantly increasing the manufacturing cost of the assembly or increasing manufacturing difficulty.